GPRS Roaming is an important aspect of modern telecommunications. Cellular devices are used more and more and not only for conversations but also for data transfer or to access services. With the advent of new generation of smart devices, like smartphones or tablet, it becomes essential that the intelligent applications installed on these devices perform flawlessly both for roaming and non-roaming scenarios. These applications usually exchange data traffic with a content server located on the internet cloud.
GPRS Roaming enables a cellular customer to access data services for pure browsing, exchanging MMS (Multi media service) for actuating applications based on data services outside the geographical coverage area of the home network.
But with the new generation of devices such as smartphones and tablets, data service is a problem when a sponsor network is used. It happens so because the APN is automatically modified by the device to the sponsor network APN which can cause the GTP traffic to fail.
Below the first time an acronym is used the full name is given between parentheses behind the acronym. Thereafter the acronym is used without given the full name.
Automatic APN (Access Point Name) modification is a problem in a roaming relationship with mobile devices such as for instance smartphones operating in GSM (Global System for Mobile Communications), UMTS (Universal Mobile Telecommunications Service) and LTE (Long term evolution) networks. The devices tend to automatically configure the GPRS (General Packet Radio Services) related parameters according to the IMSI (International Mobile Subscriber Identity) present in the SIM (Subscriber Identity Module) card which is chosen during the network acquisition process. Hence, when the IMSI that is chosen during the Location Update is not the HOME IMSI but the IMSI that belongs to another Sponsor Operator, the GPRS configurations that are activated inside the device will be with relation to the sponsor operator but NOT the client/home network. Thus the use of the sponsor network in such a scenario is the root cause of the problem. But using a sponsor network in roaming or MVNO/E scenarios is the recent trend as it allows faster implementation, ROI and has other business and operational benefits. Hence the solution to fix the issue of GPRS is mandatory as GPRS data services are not possible at present for roaming scenarios (in case of Dual/Multi IMSI solution) and for both roaming and non roaming scenarios in case of MVNO/E networks.
The above invokes problems for instance for at least the following two implementation scenarios.                Scenario 1 is in light of a Dual/Multi IMSI roaming solution where there are one or more IMSIs from other donor/sponsor networks in the SIM card (other than the basic Home IMSI) which are chosen by the mobile station to piggyback on the roaming relationships of these donors/sponsors. In such a scenario, the mobile devices such as smartphones automatically choose the APNs belonging to the sponsor network. If the HLR (Home Location Register) is able to download the Sponsor network APNs in the SGSN during the GPRS Location update process, then the GTP (GPRS Tunnelling Protocol) messages are routed to the sponsor network GGSN instead of the home network. If the sponsor network is not configured to process the GTP messages of the client network (MVNO/E or Dual/Multi IMSI customer) using their IMSIs, the GTP messages fail. If the sponsor network is configured to process the GTP messages from the home networks using their IMSIs, then still Real time charging is an issue which can only be solved by an elaborate setup (Diameter/GTP'/INAP (Intelligent Network Application Protocol)/CAMEL (Customized Applications for Mobile network Enhanced Logic) based or proprietary solution.        Scenario 2 is for a MVNO/E (Mobile virtual network operator/enabler) implementation where the MVNO/E uses the IMSI of the Sponsor network but the same MVNO/E has their own core GPRS network which includes GGSN. If a GPRS GTP message is invoked from smartphones, the GTP messages will be wrongly routed to the sponsor network (which sponsors the IMSI) and not the MVNO network.        
As stated above the problems related to GPRS service usage by devices such as smartphones are present in GSM, UMTS and LTE networks.
They are first discussed below for GSM networks for the Scenario 1.
In GSM there are several ways to establish roaming relationship between mobile operators. One way is to have direct bilateral roaming realised by direct signalling connectivity between mobile operators. In this case the Home IMSI (International Mobile Subscriber Identity) is used as an addressing parameter to identify a mobile user in a roaming network. The mobile operators need to maintain the logical or physical signalling connectivity one to one with the roaming partner networks to facilitate bilateral roaming. When there are many simultaneous roaming relationships such arrangements become difficult or unmanageable. The time taken to establish the new roaming relationships one by one will increase exponentially thus resulting in revenue loss. Another option is to connect to a roaming application and uses some other operator's IMSI, acting as a sponsor operator IMSI and piggyback on that sponsor operator's already established roaming relationship. This kind of roaming based on a sponsor IMSI is called Instant roaming based on Dual or Multiple IMSI's and is quite popular nowadays. This option is also promoted by European Union.
At present, GPRS data roaming in GSM/UMTS/LTE networks which is facilitated by the Dual/Multi IMSI solution has an issue when the transaction is invoked by some devices such as for instance a smartphone, instead of a conventional handset. Most of the smartphones have a permanent inbuilt database holding the GPRS parameters (like APN) mapped to the IMSI. So the smartphones automatically selects the GPRS data settings based on the IMSI chosen during network acquisition. For Dual/Multi IMSI SIM card, when the Sponsor IMSI is chosen at roaming location, the Sponsor network GPRS setting is chosen instead of Dual IMSI customer settings.
During the location update of the smartphone, if the HLR downloads the sponsor network APN to the SGSN in the visited PMN (Public Mobile Network) over the GSM MAP (Mobile Application Part) interface, the GTP control message is subsequently invoked during session establishment (i.e., Create PDP Context) by the SGSN over the Gn/Gp interface. But the messages are wrongly routed to Sponsor network GGSN (Gateway GPRS Support Node) instead of the Home Network GGSN. The Sponsor Network GGSN is not designed/configured to process traffic of some other networks, namely the Dual/Multi IMSI operator. Hence, GPRS data session attempts from those locations fail. This leads to a significant reduction of the reliability and customer friendliness of GPRS roaming service. Many cellular devices have this problem, such as nearly all of the present smart phones/tablets.
So to summarise, during roaming with sponsor IMSI, GPRS does not work with many devices such as smartphone and tablets which is a significant downside as most of the handsets that we see around are such devices. All the applications based on GRPS in said devices also will not work. Some devices where data roaming is essential will cease to be functional in the specific roaming location.
The smartphone or tablet manufacturers cannot change the behaviour of the handsets in the market, because there is already a high proliferation of these handsets in the market during last few years. Any significant change in the core operational logic in the handset may cost the manufacturers a fortune due to logistical requirements, interoperability issues and the cost for regression tests for each and individual model of the smartphone.
Compounding the problem is that the handset manufacturers are reluctant or unwilling to change the handset behaviour as it may well result in serious problems for the customers using conventional data roaming (not through the Multi/Dual IMSI) solutions and/or using GPRS at the home network. The GPRS settings of the handsets will change in such a case and hence their GPRS will be non usable unless they set up the settings manually. However, although many people use smartphones and similar devices only a small minority actually has the technical know-how to perform such acts.
The OS (Operating system) vendors, android/Symbian/windows/Mac cannot fix this easily by a patch or new release because of implementation issues, unknown side effects with other scenarios and also interoperability issues (handsets with the older versions). Also they do not desire to change the behaviour as it will result in serious problems for the customers using conventional data roaming (not through the Multi/Dual IMSI) solution and using GPRS at the home network. It is remarked that old versions of smartphones (prior to 2008) did not have a database to map the GRPS parameters to APN. The handset manufacturers desired to make the handsets more intelligent and be less dependent on manual configuration processes or by Over the Air SMSs invoked by the Operators. But they may not have foreseen the side effect (automatic APN modification by device) in terms of rendering GPRS service via a sponsor network.
There are some existing solutions but they all show problems of their own:                The sponsor network GGSN can be configured to process the GPRS traffic of the Dual/Multi IMSI customer: Problem with this solution is that Prepaid charging (non CAMEL (Customized Applications for Mobile network Enhanced Logic)) cannot be supported, so that the solution is not implementable. There are serious billing impacts. The Sponsor network GGSN needs to be reengineered to serve the GRRS traffic of other networks. There are also security and fraud issues.        Scripting: Some Android/Symbian scripts can be developed to force the Home GPRS settings always. However this does not work in every Android/Symbian handsets. It also does not provide a solution for IPhones, windows phones and the smartphones which does not have an OS (Samsung Star for example).        SIM applet in the SIM card can force the Home APN during power up. The time needed to acquire networks increases greatfold resulting in poor QOS (Quality of Service) which will lead to customer's complaints. Some customers use Multi IMSI SIM cards and the problem will be magnified for such situation. It is not a fool proof solution as it is not agnostic to the handset behaviour and does not work for all handsets.        
So there is a need for a solution that works universally, with all smartphones, all networks and across all categories of customers, whether prepaid or postpaid for instance.
For GPRS service for MVNOs/MVNEs (Mobile Virtual Network Enabler) similar problems occur:
Many MVNO/MVNEs do not have their own IMSI range. But they have IMSI range donated by a sponsor operator. The setup for such an MVNO can be as follows.                MVNO has the sponsor IMSI in the SIM card and defined in their profile existing in HLR        MVNO has their own Core network setup, namely the GGSN.        MVNO has their own APN and does not the sponsor operator APN for data roaming        MVNO has however defined the sponsor operator APN or a wild card in the subscriber profile in the HLR so that the VLR can permit the GTP-C Create PDP Context to be invoked with the sponsor operator APN (applicable to smartphone case).        
In this scenario, when the subscriber initiates a data session from home or from roaming location, the GTP message will be wrongly routed to the sponsor operator GGSN. This is because of the fact that the APN chosen by the smartphone will be the Sponsor Network APN and not the Home Network APN.
If the sponsor network GGSN is not configured to process the traffic from the MVNO/MVNE network, then the GTP traffic will fail. Hence GPRS service will not be possible from the devices such as smartphones.
Even if the Sponsor network GGSN is configured to process the traffic, still Real Time charging for prepaid will be quite difficult. To make it possible the Sponsor operator needs to establish a real time charging interface with the SCP of the Home Network. This also implies that the HPMN (MVNE/O) also needs to share charging related sensitive information with the sponsor which may prove to be difficult from business perspective.
Hence there is still no fool proof solution which can ward of these intricate issues and makes the deployment of the GPRS service both for the sponsor and the MVNO/E simple.
Both scenarios have the common problem that the GTP message fails because they are wrongly routed to the sponsor operator GGSN.